For example, a so-called stick-type ignition coil device directly mounted on a plug hole comprises members such as a housing, a central core, a primary coil, and a secondary coil. Of these members, the housing is cylindrical. The central core is formed like a round bar and is disposed approximately at the center of the housing. A cylindrical secondary spool is disposed at an outside periphery of the central core. The secondary coil is attached around the secondary spool. The secondary coil is formed by winding a secondary coil wire. A primary spool is disposed at an outside periphery of the secondary coil. The primary coil is attached around the primary spool. The primary coil is formed by winding a primary coil wire. A resin composition is injected into the housing so as to ensure insulation between the above-mentioned members stored in the housing and to fix the members. The resin composition is cured between the members.
The ignition coil device generates a thermal stress due to a cyclic load of heating and cooling as an engine operates and stops. That is to say, different linear expansion coefficients are attributed to the members constituting the ignition coil device and the resin composition. More specifically, linear expansion coefficients of the members such as the central core and the coil wire are larger than a linear expansion coefficient of the resin composition. This difference between the linear expansion coefficients causes a thermal stress. The thermal stress, if generated, may cause defects such as removal or crack on each member and the resin composition. Consequently, a dielectric breakdown may occur in the ignition coil device to disable an ignition plug from being supplied with a required high voltage.
For example, JP-A-H11-111547, introduces the ignition coil device injected with a resin composition having the adjusted linear expansion coefficient. According to the ignition coil device described in the document, the linear expansion coefficient of the resin composition is adjusted to a value approximating to the linear expansion coefficients of the central core, the primary coil wire, and the secondary coil wire. Because of this, a thermal stress hardly occurs due to a difference between linear expansion coefficients.
In order to decrease the linear expansion coefficient of the resin composition, it is a good practice to disperse a filler in the resin composition. However, dispersing the filler in the resin composition degrades the fluidity of the resin composition that is injected into the housing.
FIG. 6 shows an axial sectional view near the secondary coil of the ignition coil device. As mentioned above, a secondary coil 100 is attached around a secondary spool 101. The secondary coil 100 is formed by winding a secondary coil wire 102. A fine gap 108 is formed between turns of the secondary coil wire 102. The secondary coil wire 102 comprises a conductor 103 and a coat 104.
A resin composition 105 comprises a thermosetting resin 106 and a filler 107. If the filler 107 is not included, the resin composition 105 smoothly penetrates between turns of the secondary coil wire 102 through the gap 108. The resin composition 105 is cured between turns of the secondary coil wire 102 and ensures insulation for the secondary coil wire 102. The resin composition 105 hinders the secondary coil wire 102 from being wound irregularly.
If the filler 107 is dispersed in the resin composition 105, however, the filler 107 hinders the resin composition 105 from passing through the gap 108. This makes it difficult for the resin composition 105 to penetrate between turns of the secondary coil wire 102. FIG. 6 illustrates this state. Accordingly, it is difficult to ensure insulation for the secondary coil wire 102. In addition, the secondary coil wire 102 easily becomes wound irregularly.
In consideration for this, JP-A-H4-345640, introduces the coil-that ensures fluidity of the resin composition injected into the housing by widening the filler's size distribution and applying the closest packing. However, this document provides no description about a specific form of the particle size curve.